Plain English Meaning
A setting on the RNAV unit that you switch to when flying an approach. It makes the course needle more sensitive, so the aircraft has to stay tighter to the centerline than during cruise.
Full Definition
An operating mode of an RNAV system used during instrument approach procedures, in which the unit's course-deviation sensitivity is increased so that a small lateral deviation produces a larger needle deflection than in en route mode. In this mode the RNAV computer continues to display position relative to a defined , but with finer guidance suitable for .